Copter: use const reference when retrieving wpnav destination

ArduCopter/mode_auto.cpp

@@ -1524,7 +1524,7 @@ bool ModeAuto::verify_land()
             // check if we've reached the location
             if (copter.wp_nav->reached_wp_destination()) {
                 // get destination so we can use it for loiter target
-                Vector3f dest = copter.wp_nav->get_wp_destination();
+                const Vector3f& dest = copter.wp_nav->get_wp_destination();
 
                 // initialise landing controller
                 land_start(dest);

ArduCopter/mode_guided.cpp

@@ -370,7 +370,7 @@ void ModeGuided::takeoff_run()
 {
     auto_takeoff_run();
     if (wp_nav->reached_wp_destination()) {
-        const Vector3f target = wp_nav->get_wp_destination();
+        const Vector3f& target = wp_nav->get_wp_destination();
         set_destination(target);
     }
 }

ArduCopter/mode_zigzag.cpp

@@ -123,7 +123,7 @@ void ModeZigZag::return_to_manual_control(bool maintain_target)
         stage = MANUAL_REGAIN;
         loiter_nav->clear_pilot_desired_acceleration();
         if (maintain_target) {
-            const Vector3f wp_dest = wp_nav->get_wp_destination();
+            const Vector3f& wp_dest = wp_nav->get_wp_destination();
             loiter_nav->init_target(wp_dest);
             if (wp_nav->origin_and_destination_are_terrain_alt()) {
                 copter.surface_tracking.set_target_alt_cm(wp_dest.z);